US CNC Processing Service Guide

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  What matters most when you source
• 2
  Which machining route fits your part
• 3
  What a US CNC processing service guide should really tell you
• 4
  Tolerance: where ±0.005 mm is realistic and where it is not
• 5
  Material and axis count drive the quote
• 6
  Lead time, MOQ, and what a quote should include
• 7
  Certifications and inspection reports by industry
• 8
  Finishing, marking, and confidentiality
• 9
  How to vet a supplier in six steps

Most sourcing pages explain what CNC machining is. That part is settled: a program moves a cutting tool through a workpiece and removes material until the geometry matches the model. The useful question is narrower. Can the supplier you are talking to hold your tolerance on your geometry, in your material, on your date?

That question has four moving parts: tolerance, material, quantity, and paperwork. A shop can be strong on one and weak on another. A supplier that runs 16 simultaneous 5-axis centers may still be the wrong pick for a simple 6061 plate order that needs to ship in three days.

This guide is written for the engineer or buyer who already has a drawing and a deadline. We walk through the checks that separate a supplier who can do the job from one who can only quote it.

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  Start from the drawingIdentify the true critical dimensions before you talk to anyone.
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  Match the process to the geometryAxis count is a cost driver, not a quality badge.
• 3
  Confirm the paperworkCertificates and inspection reports follow the industry, not the price.

±0.005 mm (±0.0002 in) is achievable on well-supported features with a single setup. That usually means a 5-axis center or a mill-turn machine that finishes the critical face without releasing the part. On a 3-axis job with three refixtures, the same callout invites stack-up error that no amount of inspection will fix after the fact.

Surface finish travels with tolerance. A bore held at ±0.005 mm generally needs Ra 0.8–1.6 μm or better on the sealing or bearing surface, because a rough wall changes the effective fit. General clearance faces can sit at Ra 1.6–3.2 μm as machined. Fine finishes down to Ra 0.2–0.8 μm are available, but they add a finishing pass and time.

Two practical rules. First, put tight tolerance only where the function needs it. Second, tell the supplier which dimensions are critical at the quotation stage, not after the first article arrives. A shop that knows the two critical dimensions can plan the setup around them.

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  Single-setup featuresBest candidate for ±0.005 mm.
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  Refixtured featuresLoosen to ±0.02 mm unless the fixture is dedicated.
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  Thin walls under 1 mmExpect deflection; plan a light finishing pass.

Material choice changes tooling, speeds, and risk. Aluminum 6061, 7075, and 2024 cut fast and hold tolerance well. Stainless 303 and 304 are common but work-harden; 316L and 17-4PH need slower feeds and sharper tools. Titanium Ti-6Al-4V and Inconel move the job into a different cost bracket because of tool wear and heat.

Axis count should follow the geometry, not the marketing. A part with features on five faces or angled holes is a natural 5-axis job. A flat plate with a bolt pattern is not. If a quote comes back built on 5-axis time for a part that a 3-axis mill could finish, ask why.

GreatLight runs 127 high-precision CNC machines: 16 simultaneous 5-axis centers, 16 mill-turn centers, 12 four-axis mills, and 27 three-axis machines. Large-travel machines handle parts up to 4,000 mm, with a Ø400 mm rotary table for round work. That spread matters because the right machine for the part keeps the price honest.

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  Aluminum6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12.
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  Stainless303, 304, 316, 316L, 420, 430, 431, 440C, 17-4PH.
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  Titanium and specialTA1, TA2, TC4 (Ti-6Al-4V), Inconel, magnesium AZ31B / AZ91D.
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  PlasticsABS, PC, PMMA, POM, PA, PEEK, PP, HDPE, carbon fibre.

Lead time is a promise about capacity, not a wish. Ask when production starts and what triggers it. A shop that can begin within 24 hours of a released order is holding open capacity. A shop that quotes six weeks is usually batching your job behind larger runs.

MOQ is the second silent blocker. If a supplier needs 500 pieces to open a setup, a prototype program stalls. No minimum order quantity means the same supplier can run one part and a 10,000+ part run, which removes the re-qualification cost of switching shops mid-program.

A complete quote lists material grade, machine class, tolerance band, finish, inspection scope, and shipping terms. If the price arrives without a DFM note, ask for one. DFM feedback on wall thickness, tool reach, or an unnecessary tight callout often saves more than a discount.

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  Quote turnaroundQuotation and free DFM analysis within 12 hours.
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  Production startWithin 24 hours after order release.
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  ShippingParts ship in 3–5 days.

Certificates are a gate, not a ranking. General industrial work is covered by ISO 9001:2015. Automotive and EV parts usually require IATF 16949:2016, and medical device components require ISO 13485:2016. If your data package leaves the building, ISO 27001:2022 covers information security.

Inspection scope decides what you can prove at incoming QC. A supplier running 100% inspection before shipment checks raw material, monitors in process, and inspects final parts. Reports are available on request. For a first article or a regulated program, ask for the report format before the order, not after.

A qualification rate of 99.99% is a useful benchmark, but the number is only meaningful alongside the inspection method. Ask which instruments measure your critical dimension and how the result is recorded.

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  ISO 9001:2015General machining programs.
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  IATF 16949:2016Automotive and EV parts.
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  ISO 13485:2016Medical device components.
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  ISO 27001:2022Handling of sensitive drawings and data.

Finishing is where cosmetic and functional requirements split. Anodizing (clear, colour, hardcoat, conductive) and plating change dimensions slightly, so the machinist needs to know the finish before setting the tolerance. Powder coating and black oxide are thicker still. Bead blasting, tumbling, brushing, and polishing change surface texture without adding much thickness.

Laser marking and engraving handle part numbers, lot codes, and UDI text. Minimum character height is 1.5 mm, which is worth checking against your label spec before release. Small text on a curved surface is a common source of rework.

Confidentiality is part of the sourcing decision. Uploads are handled as secure and confidential, and an NDA is available on request. For defense-adjacent or unreleased products, settle this before sending the model.

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  Finish before toleranceTell the shop the coating thickness when you release the drawing.
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  Marking height1.5 mm minimum character height.
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  NDAAvailable on request before files are shared.